A pulse of light has an optical spectrum that is inversely related to the duration of the pulse. As the duration of the pulse decreases, the width of the optical spectrum increases. Thus, a relatively short pulse has a relatively wide optical spectrum.
Conventional optical scanning systems are optimized for monochromatic or near monochromatic light. The width of the optical spectrum of monochromatic or near monochromatic light is negligible relative to the chromatic aberrations within the optics of the system and the dispersive effects of the optical materials from which the optical elements are constructed. In contrast, ultra-short pulse optical sources generate ultra-short pulses of light. Such an ultra-short pulse results in an optical spectrum having a width that is non-negligible relative to the chromatic aberrations and dispersive effects within the optics of the system.
Chromatic aberrations of optics can affect the temporal and spatial profiles of ultra-short pulses. For example, chromatic aberrations of a scan lens can vary versus a scan angle in a scanning optical system. Conventional laser scanning optical systems are designed to direct and focus narrowband or single frequency laser light because the laser modulation rates in these systems are much lower than with ultra-short pulse lasers. As a result, chromatic aberrations and material dispersion have a negligible impact on such systems.
In contrast, because of the relatively wide, spectrum of ultra-short pulses, ultra-short pulses may be adversely affected by the chromatic aberrations and material dispersion. As a result, an ultra-short pulse will be distorted both spatially and temporally after passing through a conventional laser scanning optical system.
Because of the effects of chromatic aberrations, some ultra-short pulse scanning optical systems are limited to scanning through a very small scan angle if at all. Alternatively, the entire optical system or the object being illuminated is moved such that distortions due to scanning are not introduced. However, because of the bulk of the entire optical system or the object being illuminated, the scan rate is limited.
Accordingly, there remains a need for an improved scanning optical system capable of scanning over a larger image field at a faster scan rate.